The New Jersey Democrat says he's concerned that hard-fought progress in several areas such as civil rights, environmental protection, education and criminal justice could be wiped out by the new administration.

"We have to let folks know we're going to fight the good fight here," Booker said in an interview in his Washington office. "This year could potentially be one of the most difficult and challenging years in modern American history."

It's a sudden turn of events for a lawmaker who arrived at the U.S. Capitol with a reputation for liking the spotlight but instead sought to hide from its glare, working quietly with members of both parties to advance legislation and using his celebrity status to help elect more Senate Democrats.

Booker said the entire Senate Democratic caucus plans to speak out, but he is in a different category than many of his colleagues. At age 47, he's much younger than the Democratic leaders in Congress or the party's last presidential nominee.

"There will be a passing of the torch," Democratic consultant Glenn Totten said. "Senator Booker is a very good choice."

Booker brings more than just the voice of a new generation. He has 2.1 million Twitter followers and another 148,000 on Instagram.

"He probably has more national name recognition than any junior senator in America," said Matthew Hale, a political science professor at Seton Hall University. "That alone makes him speaking out against Trump a big deal."

Some Democrats welcomed Booker to the fight.

"Cory's no longer the celebrity, he's a genuine future contender," said U.S. Rep. Bill Pascrell Jr. (D-9th Dist.). "He's ready for prime time. He's articulate, he knows what he's talking about and he does his homework."

"This is a calculated move on his part to be the new voice of the Democratic Party," Republican consultant Craig Stevens said.

Indeed, Booker's role now could fuel a future White House run, said Julian Zelizer, a professor of history and public affairs at Princeton University.

"If he emerges as a leader of the opposition, Booker will be in good shape to be a top name for 2020," Zelizer said. "The role won't be easy since Trump will come at his opponents with everything he has. But if Booker can withstand it, he might emerge as a popular candidate for the next election."

Booker already is being scrutinized by potential allies. Supporters of U.S. Sen. Bernie Sanders (I-Vt.) criticized him for voting against allowing imports of prescription drugs from foreign countries.

"This vote is a disgraceful betrayal of every patient and consumer in America," said RoseAnn DeMoro, executive director of National Nurses United, which ran a pro-Sanders super political action committee.

Several major drug companies, including Johnson & Johnson and Merck & Co., call New Jersey home. In his 2013 special and 2014 general elections, Booker received $233,750 from employees of drug manufacturers, more than any other congressional candidate, according to the Center for Responsive Politics.

Besides Booker, 12 other Democrats, including U.S. Sen. Robert Menendez (D-N.J.), opposed the amendment, even as they supported two other provisions related to prescription drugs. Booker said he supported the importation of prescription drugs but the proposal lacked "consumer protections that ensure foreign drugs meet American safety standards."

Republicans said Booker could be risking his reputation as a lawmaker willing to reach across the aisle.

Booker's decision to oppose Sessions didn't sit well with many Republicans.

"He was looking to jump start talking about 2020," GOP consultant Chris Russell said. "I thought this was a misstep. If he still wants to be this post-partisan guy, he's doing damage to the image he's trying to build."

And U.S. Sen. Tom Cotton (R-Ark.) wrote on Facebook: "Senator Booker is better than that, and he knows better."

Booker said he won't be quiet.

"People are going to criticize you no matter what you do, so do what's right," Booker said. "If people want to speculate or criticize or condemn, so be it. I'm taking steps I believe are the right things to do."